Seaweed derived compounds are a renewable resource utilised in the manufacturing and food industry. This study focuses on an Enriched seaweed extract (ESE) isolated from Ascophyllum nodosum. ESE was screened for antiviral activity by plaque reduction assays against IAV H1N1 and H3N2 subtypes. Time of addition assays and FACS analysis were used to help determine the mode of action. The therapeutic potential of ESE was then explored using differentiated human bronchiole epithelial cells at the air liquid interphase and a murine model challenged with IAV. The data indicates ESE primarily interacts directly with virions, preventing virus cell binding. Interestingly, ESE also inhibits early and late stage of the influenza lifecycle when treatment occurs after cell binding. This inhibitory effect appears to prevent internalisation of virus and release of progeny virus by targeting neuraminidase activity. Intranasal administration of ESE in mice infected with IAV reduced viral load in lung tissue. ESE may be a promising broad acting antiviral agent in the treatment of influenza infections.

Phlorotannin rich Ascophyllum nodosum seaweed extract inhibits influenza infection